Tailoring the Rheology of Dynamic Covalent Composites with Particle Size and Bond Lifetime

The mechanical properties of dynamic polymer networks are known to depend on the time-average crosslink density, bond lifetime, and donor/acceptor stoichiometric balance. However, it is unclear how these factors map to composites where particles dispersed in a polymer melt can form dynamic bonds at the filler-matrix interface. Here, we use these dynamic covalent composites comprising thiol-coated silica particles and ditopic benzalcyanoacetamide-based Michael acceptors to understand how their steady and oscillatory shear response depends on particle size. While larger particles form flowable composites which exhibit negative thixotropy, decreasing particle size suppresses shear hysteresis and leads to solid-like samples due to an increased dynamic crosslink density. Utilizing dynamic bond lifetimes measured for small molecule analogs, we also explore how the electron-donating/withdrawing nature of the thia-Michael acceptor influences stress relaxation in these dynamic composite networks. We anticipate these insights will help integrate dynamic covalent chemistries and polymer composites into stress-responsive smart materials.